Fact Sheet ST-572October 1994

Robinia pseudoacacia ‘Purple Robe’

Figure 1. Middle-aged ‘Purple Robe’ Black Locust.

‘Purple Robe’ Black Locust 1

Edward F. Gilman and Dennis G. Watson2

INTRODUCTION

This cultivar of Black Locust probably grows toabout 40 feet tall with a spread of 20 to 35 feet (Fig.1). The upright, rounded growth and short, irregularbranches form a tighter canopy than the species andcast medium shade below the tree, allowing a lawn togrow. The 6 to 14-inch-long, dull, blue/green leaves,made up of multiple leaflets, are some of the last toappear in spring and often drop early in the autumn,just barely fading to a sickly yellow/green beforedropping. For approximately a 10-day period in latespring, the trees are festooned with four to eight-inch-long, dense clusters of extremely fragrant, one-inchrose to pink blossoms (similar to sweet-peas) whichare literally "alive" with the bustling activity ofvisiting bees. The honey which is produced from thespecies is quite delicious and sought-after. The two tofour-inch-long, dark red to black, leathery seeds podswhich follow will persist on the trees throughout thewinter.

GENERAL INFORMATION

Scientific name: Robinia pseudoacacia‘Purple Robe’Pronunciation: roe-BIN-nee-uhsoo-doe-uh-KAY-shuhCommon name(s): ‘Purple Robe’ Black Locust,‘Purple Robe’ Common LocustFamily: LeguminosaeUSDA hardiness zones: 4 through 8 (Fig. 2)Origin: native to North AmericaUses: reclamation plant; shade tree; no proven urbantolerance

Availability: somewhat available, may have to go outof the region to find the tree

DESCRIPTION

Height: 30 to 50 feetSpread: 20 to 35 feetCrown uniformity: irregular outline or silhouetteCrown shape: oval; uprightCrown density: openGrowth rate: fastTexture: fine

Foliage

Leaf arrangement: alternate (Fig. 3)Leaf type: odd pinnately compoundLeaflet margin: entire

1. This document is adapted from Fact Sheet ST-572, a series of the Environmental Horticulture Department, Florida Cooperative Extension Service,Institute of Food and Agricultural Sciences, University of Florida. Publication date: October 1994.

2. Edward F. Gilman, associate professor, Environmental Horticulture Department; Dennis G. Watson, associate professor, Agricultural EngineeringDepartment, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville FL 32611.

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Leaflet shape: elliptic (oval); oblong; obovate

Figure 2. Shaded area represents potential planting range.

Leaflet venation: pinnateLeaf type and persistence: deciduousLeaflet blade length: less than 2 inchesLeaf color: blue or blue-green; greenFall color: yellowFall characteristic: not showy

Flower

Flower color: lavenderFlower characteristics: pleasant fragrance; springflowering; very showy

Fruit

Fruit shape: podFruit length: 3 to 6 inches; 1 to 3 inchesFruit covering: dry or hardFruit color: black; redFruit characteristics: attracts birds; attracts squirrelsand other mammals; inconspicuous and not showy;fruit, twigs, or foliage cause significant litter; persistenton the tree

Trunk and Branches

Trunk/bark/branches: grow mostly upright and willnot droop; showy trunk; should be grown with a singleleader; thorns are present on the trunk or branchesPruning requirement: requires pruning to developstrong structureBreakage: susceptible to breakage either at the crotchdue to poor collar formation, or the wood itself isweak and tends to breakCurrent year twig color: brownCurrent year twig thickness: thinWood specific gravity: 0.69

Culture

Light requirement: tree grows in part shade/part sun;tree grows in full sunSoil tolerances: clay; loam; sand; acidic; alkaline;well-drainedDrought tolerance: highAerosol salt tolerance: high

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Other

Figure 3. Foliage of ‘Purple Robe’ Black Locust.

Roots: surface roots can lift sidewalks or interferewith mowingWinter interest: tree has winter interest due tounusual form, nice persistent fruits, showy wintertrunk, or winter flowersOutstanding tree: not particularly outstandingInvasive potential: seeds itself into the landscapeVerticillium wilt susceptibility: susceptiblePest resistance: very sensitive to one or more pestsor diseases which can affect tree health or aesthetics

USE AND MANAGEMENT

These seeds are widely dispersed by birds andother wildlife, and along with the root suckering,invasive root system, Black Locust can spread intosurrounding landscapes. This feature, along with thethorns found along the branches, should be consideredwhen placing Black Locust in the ornamental garden.It is probably best saved for the reclamation project orroadside planting where trees receive no maintenance.If left to its own devices, Black Locust will formdense thickets, even on the poorest soils, a fact whichmakes it quite useful in reclamation applications.

Although the wood of Black Locust is reputed tobe extremely strong and durable (pioneers used it tofashion nails for building ships and houses), thebranches of the species are brittle and subject todamage in high winds. This may be due largely to thetendency for the branches to form a narrow angle withthe trunk and to grow aggressively relative to the trunkforming embedded bark. This can be partiallycorrected by pruning the major branches so they growto no more than about half the size of the trunk. Thiswill probably not be as much a problem on thiscultivar as it is on the species.

Able to tolerate drought, salt, and poor soil, BlackLocust will grow in full sun or partial shade on almostany soil acid or alkaline except that which ispermanently wet. A tough tree which should be savedfor the toughest sites. Not for general use in urbanareas due to borer problems unless located in a moist,well-drained site with plenty of soil space.

Other cultivars include: ‘Erecta’, upright form;‘Frisia’ - yellowish leaves in early summer.

Propagation is by cuttings.

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Pests

Locust borer is a serious pest although borer-resistant clones are now being developed. Alsobothered by carpenterworm, locust leaf miner, andscales. Leaf miner is a universal problem on thespecies. Trees along the highways in the south can beseen in summer riddled with damage from this pest.

Diseases

This tree is susceptible to canker, leaf spot, andpowdery mildew.